For my Physics 123 class, each team was to come up with a group project somehow related to physics in which we would research a topic, build something to perform experiments with (optional), write a paper, and present our project to the class at the end of the quarter. We decided on building a trebuchet small enough that we could fire it in a fairly large lecture hall. But what to use for projectiles? We needed something that wouldn't hurt any one or damage school property if errant shots when flying where they weren't supposed to. We quickly settled on marshmallows over a lunch meeting. They are light and soft, two properties not typically found in projectiles so it would be a good challenge to see how far we could hurl them. We set out a goal to launch them 20-30 feet, assigned roles to team members, and turned in our project proposal which was quickly approved.
My part of the project was the design, fabricate, and build portion. Other team members were tasked with researching the history and present day uses of the trebuchet and to make an attempt at working out the physics and math of the trebuchet. The math proved to be one of the most difficult aspects of the project since there is a whipping action where the fixed end of the whip is moving through an arc path while the sling accelerates through it's own path.
This instructable will focus on our design selection and the actual design we built.
Step 1: A Little History & Design Selection
After some initial research to see what different styles of trebuchet there were, we set about deciding on what to build. The original trebuchet were based on the sling staff, basically a human powered trebuchet. The Chinese developed the traction trebuchet in approximately 400 BC which was a large lever arm and a sling for projectiles. The force to hurl the projectile was generated by many people simultaneously pulling down on ropes connected to the lever arm.
A long time later in approximately 1100AD, trebuchet with swinging counterweights arrived on battlefields. The hanging mass provided gravitational potential energy which would be relatively the same for every fire providing much greater accuracy. The swinging pendulum action reportedly brought the trebuchet to a stop faster and caused less wear and tear on the machine.
The trebuchet was the seige machine of choice for centuries but faded out of use around 1400AD with the advent of black powder and cannons. Fast forward to present day and they have become one of the tools of choice for launching pumpkins at the Punkin' Chunkin' festival, as well as projects for DIYers, backyard engineers, and physics & engineering students.
I had seen a trebuchet used for Punkin' Chunkin' called Merlin and was partial to that design. We initially looked at building that design in a scaled down version but due to the complexity, difficulty of the math involved, and limited time frame, it was eventually ruled out. The floating arm trebuchet was also considered but ruled out for the same reasons. We found an interesting design that was invented by some high school students and refined by their professor L.D. Vance known as the Murlin (creative naming huh?) which stands for multiple radius-linear node. The design features a hanging mass connected to a rope wrapped around multiple decreasing length arms. As the weight falls, the rope pulls faster due to acceleration and due to a changing arm length ratio. This design seemed very effective with Vance's golfball flinging version hitting a range of 636 feet. It was also simple enough for us to tackle in our time frame so we moved forward with the design phase.
Videos of Inspirations for Our Potential Designs
Step 2: Design & Build Process
Step 3: Test Firing & Troubleshooting
Step 4: Conclusion
We exceeded our original goal of 20-30 feet with a longest recorded launch of 52.5 feet and launch velocities upwards of 45 MPH. Comparison between the kinetic energy of the projectile and the gravitational potential energy of our hanging mass showed an efficiency of a little over 16%. We feel this can be greatly improved upon with the following tweaks:
- Replace stretchy rope with Spectra or similar low to no stretch line.
- Improve sling/pouch design.
- Replace or modify firing fingers with design that allows ring to slide off smoothly and consistently.
- Leave marshmallows in a paper bag for several days (they get much harder).
- Eliminate slop in frame design.
- Stake frame down to the ground.
- Replace plastic bushings from arm with better bushing/bearing arrangement.
- Increase pulley diameter.
- Increase height of hanging mass to ensure potential energy remains for full travel of arm.
- Improve launch angle to approach or hit 45°.